Method of improving magnetic permeability of cube-on-edge oriented silicon-iron sheet stock

ABSTRACT

The method of improving the magnetic properties of conventional cube-on-edge oriented, silicon-iron sheet stock by providing the sheet with a light electropolish before or after the final hightemperature orientation anneal sufficient to substantially eliminate large microscopic peaks but insufficient to significantly enhance average surface smoothness.

United States Patent Benford et a1.

[54] METHOD OF IMPROVING MAGNETIC PERMEABILITY OF CUBE-ON-EDGE ORIENTED, SILICON-IRON SHEET STOCK [72] inventors: James G. Beniord, Monroeville Boro; Ed-

ward B. Stanley, Washington Township,

Westmoreland, both of Pa.

[73] Assignee: United States Steel Corporation [22] Filed: Nov. 10, 1969 [21 Appl. No.: 875,562

[52] US. Cl ..204/140.5, 148/110, 148/112,

204/140 511 int. Cl. ..C23b 1/00,C23b 3/06 5s FieldoiSearch ..2o4/140, 140.5

[ 51 Feb. 22, 1972 Primary Examiner-John H. Mack Assistant Examiner-T. Tufarieilo Attorney-Forest C. Sexton ABSTRACT The method of improving the magnetic properties of conventional cube-on-edge oriented, silicon-iron sheet stock by providing the sheet with a light eiectropolish before or after the final high-temperature orientation anneal sufficient to substantiaiiy eliminate large microscopic peaks but insufficient to significantly enhance average surface smoothness.

4 Claims, No Drawings METHOD OF IMPROVING MAGNETIC PERMEABILITY OF CUBE-ON-EDGE ORIENTED, SILICON-IRON SHEET STOCK BACKGROUND OF THE INVENTION Recent advancements in the electrical steel art have revealed a new approach to improving the magnetic properties of ferromagnetic materials, such as silicon-iron sheet stock, showing that surface smoothness is a substantial factor in the materials magnetic permeability. Specifically, it has been shown that permeability can be enhanced for a given sheet material if the surface has a smoothness of about 5 microinches or smoother. To this end, pertinent publications teach cold reducing the sheet with very smooth rolls toeffect the surface smoothness of 5 microinches or less. For'example, see US. Pat. No. 3,347,718. Although this procedure is successful in providing an improved permeability, the process is expensive because such extreme smoothnesses are not easily maintained on the work rolls of commercial rolling mills, requiring greatly increased roll maintenance costs.

SUMMARY OF THE INVENTION This invention is predicated upon our discovery that an excube-on-edge grain oriented, silicon-iron sheet stock not only improves magnetic permeability by a very limited polishing action, but contributes to texture development to enhance the desired grain orientation. In this-process, extreme smoothness of the finished sheet is not desired since optimum results are obtained at average smoothnesses of about 8-18 microinches which is not substantially smoother than commercial oriented on-edge grain-oriented, silicon-iron sheet stock whereinthe sheet is given an exceptionally light electropolish which, although not substantially increasing average surface smoothness, does increase magnetic permeability and preferred orientation.

DESCRIPTION OF THE PREFERRED EMBODIMENT The starting'material used in the process of this invention is a conventional intermediate silicon steel containing from 2.5 to 3:5 percentvsilicon, which has been processed by conventional procedures for a cube-on-edge grain-oriented-siliconiron sheet. Typical composition objectives for such a steel are substantially as follows:

Silicon 2.5 to 3.5% Carbon at least 0.010% Sulfur at least 0.010% Manganese 0.06 to 0.12% Phosphorus up to 0.010% Nitrogen up to 0.010%

The cold rolling procedures utilized in the process of this invention may be substantially the same as those used in the prior art. That is, the hot rolled band, having a thickness of about 0.08 inch, is first cleaned to remove mill scale and then cold rolled to final gage, i.e., 0.011 to 0.014 inch, usually in two cold reductions with intermediate annealing. The procedures we refer to are well known to those skilled in the art.

' Although we find the above cold rolling procedure to be ideally suited to this process, other two or three step cold reductions maybe used. More important than the number of cold reductions and the extents thereof is that after each cold reduction the sheet must be annealed not only to relieve internul stresses induced by the cold working, but more imporceptionally light electropolish on the surface of conventional J tantly to effect a fine grained primary recrystallization structure development prior to the final anneal. Although batch annealing may be used in place of continuous annealing, we prefer continuous annealing because it provides more uniform heating and allows afaster annealing cycle to minimize exaggerated graingrowth. We therefore prefer our intermediate anneals to be continuous anneals at temperatures within the range 1,400 to l,700 F. in a protective atmosphere. The annealing time must be sufficient to completely effect primary recrystallization. Prolonged heating, however, should be avoided so that grain growth is minimized. If batch annealing is used, it is'advisable to avoid temperatures in excess of 1 ,750 F. to eliminate. the possibility of extensive grain growth.

Decarburization is usually effected during the last intermediate anneal to reduce the carbon content of the sheet to levels of 0.004 percent or less-For optimum results we prefer to anneal at a temperature within the range 1,400 to l,550 F., or ideally at about l,475 F. for a period of about 3 to 5 minutes in a moist atmosphere containing hydrogen having a dew point of +70 F.

As in other prior art processes, the sheet is given a final high temperatureanneal after the cold rolling and intermediate annealingsteps. This final high temperature anneal, or orientation anneal, is preferably a box anneal ata temperature of from 2,000 to 2,150 F. which effects a secondary recrystallization yieldingthe desired oriented grains. The annealing time which'may vary from 1 to 35 hours, must, of course, be sufficient to effect complete secondary recrystallization (grain orientation) and sulfur removal. As expected, the annealing time required is inversely proportional to annealing temperature so that higher temperatures will require shorter annealing times.

The above procedures are substantially like those frequentlypracticed in the prior art. The crux of this invention resides in a step whereby the sheet is given a light electropolish either before or after the final high temperature anneal. Optimum results are achieved if the electropolishing is effected before the final anneal. Electropolishing may be performed either in a batch process or continuous process in accordance with well known electropolishing procedures. For example, a suitable light polish can easily be effected in a period of 2 to 3 minutes in a chromic acid-phosphoric acid bath at a current density of 310 amp./ft.". The electropolishing timecan of course be reduced by using higher current densities.

The primary objective of the electropolishing step is not only to effect a slightly smoother surface, but also to properly condition the sheet surface to effect better texture development'during the'secondary recrystallization 1n order to derive the benefit of this later objective, it is obviously necessary that the sheet be electropolished prior to the high temperature final anneal wherein grain orientation is effected. In this regard, the electropolish serves to remove any surface or near surface oxides that might interfere with the secondary recrystallization process either by obstructing the motion of .grain boundaries or by serving as sources of oxygen potential that might unfavorably alter the surface energy relationships of grains of various orientations.

Although optimum results can be achieved only if the sheet is electropolished prior to the final anneal as explained above, some of the benefits of this invention can be realized by electropolishing the oriented sheet after the final anneal. Since the sheet has already undergone secondary recrystallization after .the final anneal, the electropolishing can no longer enhance less provide significant improvements in magnetic permeability with little or no actual change in average roughness. To be more specific, commercially produced oriented sheets seldom have average roughnesses above about 20 microinches, and

0.018 percent sulfur, and 3.26 percent silicon. All samples were identically processed to 0.0l2-inch thick oriented sheet substantially as described above except that samples 2 were electropolished after the high temperature final anneal carbon, 0.08 percent manganese, 0.006 percent phosphorus,

typically have average roughnesses in the range 10 to 20 5 whereas samples 1 were not electropolished. Electropolishing microinches. After the light electropolish of this invention, the of samples 2 were effected for 2 minutes in a chromic acidsheets may still have average roughnesses in the 8 to 20 phosphoric acid bath 100 grams chromic acid and 532 ml. microinch range, but yet have substantially improved magphosphoric at a current density of 310 amp./ft. and at 25 netic permeability. It is obvious therefore that our elecvolts. No measurements of surface roughness were made tropolishing step does provide benefits other than those nor- 10 because the magnesium silicate glass formed on the fully mally expected from merely rendering a smoother surface. processed material precludeda r iganingful measurement.

To explain the above phenomenon, we have learned that TABLE! magnetic permeability is not so much a function of surface smoothness, but more importantly is a function of the large microscopic peaks which are usually randomly dispersed over M I erage the surface of cold rolled sheet steels. The larger such peaks samples permeabimy and the greater their number, the lower the sheets magnetic permeability will be. Hence, our electropolishing treatment NO, Su f Treatment 10kg, 15 kg, 17 kg. may not and need not actually change the average sheet roughness as measured by a Surfindicator," but it will 20 1 Notmwmpofished 3266, 25846 8925 eliminate or reduce the large microscopic peaks. Although eX- 2 Electropolished 36576 29152 9146 cessive electropolishing to produce a substantially smoother surface does also eliminate such peaks, this also removes substantial amounts of surface metal unnecessarily which serves Table ll below is substantially the same as Table l except only to reduce recovery and efficiency without any improvethat for these tests, the electropolishing was performed prior ment in permeability. to the high temperature anneal. The chemistry of these heats it is apparent, therefore, that in our process all that is and the electropolishing procedures were identical to those required is that the sheet be subjected to a light electropolish shown above. Roughness measurements were made on these regardless of the average surface roughness before or after the samples since both had essentially clean surfaces. Magneticpolish. it follows that a sheet electropolished according to this torque measurements on the nonelectropolished samples invention will have a higher magnetic permeability than will a revealed 86 percent texture development, whereas, the eleclike sheet not so polished even though the nonpolished sheet tropolished samples were developed to 92 percent.

TABLE 11 Surface roughness Samples Arithmetic Peaks per Average permeability average inch over 50 N0 Surface treatment (microinches) microinehes 10 kg. 16 kg. 17 kg.

Not electropolished--. 11-13 286 31,500 27,778 9,197 Eleetropolished 8-11 71 34, 157 32,061 14,295

may have a smoother surface. From the above tables, it will be readily apparent to those The electropolishing procedures themselves may be Subskilled in the art that the process of this invention is capable of stantially the same as those known and practiced in the prior producing cube-on-edge grain oriented, silicon-iron sheet art. Although any person skilled in the art could therefore stock having an improved magnetic permeability. readily determine a variety of suitable electropolishing we claim, parameters we have been most Succesf.ul i chromic 1. A method for increasing the magnetic permeabilitv of acid-phosphoric acid bath at current densities wrthm the range conventional cube on edge grain oriented silicondron Sheet to 1,000 within this range we f preferred a stock containing from 2.5 to 3.5 percent silicon comprising cun'em density of about 440 P- at Pollshmg of lightly electropolishing the sheet surfaces sufficient to subfl'om 30 to 120 seconds with olmmum results at about 120 stantially eliminate large microscopic peaks, but insufficient seconds. For example, a given heat of steel was processed to to appreciably enhance average Surface smoothness to grain oriented sheet without any electropolishing and had a thereby render a Surface smoothness of from 8 to 20 magnetic permeability of 9,200 at 17 kilogauss. An identical microinches. sheet electropolished for 30 seconds at 440 ampJft. prior to In the method for producing cube on edge grain Oriented the final anneal but Otherwise identically Processed had a silicon-iron sheet stock wherein a steel containing 2.5 to 3.5 magnetic Permeability of 11,500 at 17 ki1gau5S- Extending percent silicon is cold reduced to 0.011 to 0.0l4 inch in a sethe eleclmpolish to 120 Seconds resulted in a magnetic ries of cold rolls with intermediate annealing to effect primary -l y. 12,700 at 17 kilogau5s- An average roughness recrystallization following each cold roll, and finally annealing f 10 I0 13 mim'oil'mhei was not pp y Changed by either at high temperatures to effect secondary recrystallization electropolish Extending the electropolish beyond 120 orientation, the improvement comprising electropolishing the Seconds did not pp y effect Permeabimy but merely sheet surfaces after cold rolling sufficient to substantially resulted in ex siv metal removal eliminate large microscopic peaks, but insufficient to ap- Tables 1 and below are presented to better illustrate the reciably enhance average surface smoothnes5 detailed advantages of this invention. Table 1 shows the 3 The method f claim 2 in which said electropolishing is average improvement in magnetic permeability values for a performed befo h fina] hi h4 l, given heat f Or n Sheet at 15 and 17 The heat 4. The method of claim 2 in which said electropolishing is shown in Table I had the following chemistry: 0.022 percent performed fte h fi l hi h-t m rat nn aL 

2. In the method for producing cube-on-edge grain oriented silicon-iron sheet stock wherein a steel containing 2.5 to 3.5 percent silicon is cold reduced to 0.011 to 0.014 inch in a series of cold rolls with intermediate annealing to effect primary recrystallization following each cold roll, and finally annealing at high temperatures to effect secondary recrystallization orientation, the improvement comprising electropolishing the sheet surfaces after cold rolling sufficient to substantially eliminate large microscopic peaks, but insufficient to appreciably enhance average surface smoothness.
 3. The method of claim 2 in which said electropolishing is performed before the final higH-temperature anneal.
 4. The method of claim 2 in which said electropolishing is performed after the final high-temperature anneal. 